2011-11-17 02:04:27 +08:00
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<HR>
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<H3>fix append_atoms command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID append_atoms face arg ... keyword value ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>append_atoms = style name of this fix command
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<LI>one face/arg pairs must be appended
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<LI>face = <I>zhi</I>
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<LI>zero or more keyword/value pairs may be appended
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<LI>keyword = <I>size</I> or <I>freq</I> or <I>temp</I> or <I>random</I> or <I>units</I>
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<PRE> <I>size</I> args = Lz
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Lz = z size of lattice region appended in a single event(distance units)
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<I>freq</I> args = freq
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freq = the number of timesteps between append events
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<I>temp</I> args = target damp seed extent
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target = target velocity for region immediately ahead of the piston
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damp = damping parameter (time units)
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seed = random number seed for langevin kicks
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extent = extent of thermostated region (distance units)
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<I>random</I> args = xmax ymax zmax seed
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<I>xmax</I>, <I>ymax</I>, <I>zmax</I> = maximum displacement in particular direction (distance units)
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<I>seed</I> = random number seed for random displacement
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<I>units</I> value = <I>lattice</I> or <I>box</I>
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<I>lattice</I> = the wall position is defined in lattice units
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<I>box</I> = the wall position is defined in simulation box units
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 1 all append_atoms zhi size 5.0 freq 295 units lattice
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fix 4 all append_atoms zhi size 15.0 freq 5 units box
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fix A all append_atoms zhi size 1.0 freq 1000 units lattice
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>This fix creates atoms on a lattice, appended on the zhi edge of the system box.
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This can be useful when a shock or wave is propagating from zlo. This allows
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the system to grow with time to accommodate an expanding wave. A simulation
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box must already exist, which is typically created via the
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<A HREF = "create_box.html">create_box</A> command. Before using this command, a
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lattice must also be defined using the <A HREF = "lattice.html">lattice</A> command.
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</P>
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<P>This fix will automatically freeze atoms on the zhi edge of the system, so that
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overlaps are avoided when new atoms are appended.
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</P>
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<P>The <I>size</I> keyword defines the size in z of the chunk of material to be added.
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</P>
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<P>The <I>random</I> keyword will give the atoms random displacements around their
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lattice points to simulate some initial temperature.
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</P>
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<P>The <I>temp</I> keyword will cause a region to be thermostated with a Langevin
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thermostat on the zhi boundary. The size of the region is measured from zhi and
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is set with the <I>extent</I> argument.
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</P>
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<P>The <I>units</I> keyword determines the meaning of the distance units used
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to define a wall position, but only when a numeric constant is used.
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A <I>box</I> value selects standard distance units as defined by the
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<A HREF = "units.html">units</A> command, e.g. Angstroms for units = real or metal.
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A <I>lattice</I> value means the distance units are in lattice spacings.
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The <A HREF = "lattice.html">lattice</A> command must have been previously used to
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define the lattice spacings.
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</P>
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<HR>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
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are relevant to this fix. No global or per-atom quantities are stored
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by this fix for access by various <A HREF = "Section_howto.html#4_15">output
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commands</A>. No parameter of this fix can be
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used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A> command.
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This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
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</P>
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<P><B>Restrictions:</B>
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</P>
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2011-11-17 02:35:55 +08:00
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<P>This fix style is part of the SHOCK package. It is only enabled if
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LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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2011-11-17 02:04:27 +08:00
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<P>The zhi boundary on which atoms are added with append_atoms must be shrink-wrapped.
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The zlo boundary may be any boundary type other than periodic.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "fix_wall_piston.html">fix wall/piston</A> command
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</P>
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<P><B>Default:</B> <I>size</I> = 0.0, <I>freq</I> = 0, <I>units</I> = lattice.
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</P>
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<HR>
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</HTML>
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